Drying must be done at a low temperature F.) in comparatively damp air (5 to 8o per cent) to avoid excessive tenderness of the gelatine and also permanent distortion of the film, which would render its employment difficult.
The film, which expands slightly in the baths, contracts on drying, its shrinkage varying from o-o5 per cent to per cent, according to the nature of the base and the drying conditions. This shrinkage increases with age and can exceed i per cent, especially if the film is stored in a very dry atmosphere.
904. Titles. The various scenes of a film are generally preceded by titles and sub-titles, printed from type or made from drawn designs, occasionally combined with still or animated pictures.
To enable titles to be read satisfactorily, it is usual to allow 4/5ths of a second per word, or 10 in. of standard film, with a minimum of 20 in. per title.
These titles are generally taken, by means of a special camera, on positive film, by copying opaque texts illuminated by reflected light, or transparent texts illuminated from behind, either in diffused or " directed " light.
To avoid dazzle, titles are always projected with white letters on a black ground. The films for them are obtained either directly or by printing under a negative taken under appro priate conditions.
905. Joining Negatives. To permit automatic printing and development of the positive film and to avoid the joins in it which are sometimes the cause of trouble during projection, the nega tives of the various scenes are generally (after trial positives have been printed, examined, cut, and assembled) joined by cementing into one single negative film.
906. Printing of Positives. Cinematograph films are printed in machines with an inter mittent action, the printing being done frame by frame,' or in continuous printers, the nega tive and positive passing together behind a slit, the opening of which adjusts the duration of exposure.
Before printing, a test is usually made to ascertain the working conditions which will give the best result. This test is indispensable when development is carried out for an almost fixed time, as is the case with certain types of con tinuous printers. In modern installations, use is made for this purpose of automatic testers, which afford about io frames of each scene, each with a given exposure such as may be obtained by inserting suitably graded resistances in the circuit of the lamps.'
After ascertaining the best conditions, the negative is usually nicked on its edges, at a point marking the common limit between two scenes, the passage of these nicks releasing in the printer a mechanism which automatically adjusts the intensity of the lamps (intermittent printers) or the duration of the exposure (con tinuous printers).
Trial positives and small numbers of film lengths are developed under the same conditions as described for negatives. The developed, fixed, and washed films are sometimes subjected to various toning processes or to general tinting.
907. Storage and Maintenance of Positive Film. Films cannot retain the necessary flex ibility, especially after the considerable heating which they undergo when unless they are kept, when not in use, at as low a temperature as possible and in comparatively moist air. They can with advantage be placed from time to time in a moistening box, i.e. a box with a perforated double bottom. In the upper compartment is the film, loosened into a slack coil, and in the lower compartment, a container filled with water or with wet pads.
Before using a new film, it is advisable to put paraffin wax on its edges, an operation which is performed by special machines. Through the combined action of heat and friction, the wax is sufficiently softened to form a lubricant which decreases the resistance to the passage of the film through the projector. This is very neces sary, as the gelatine of a new film still retains a fair amount of moisture, and therefore is liable, when passing through the gate, to form lumps which give rise to scratches and which brake the film and thus can cause the toothed drums to cut the perforations.
Scratches on films which have been much used, and carelessly, give a very unpleasant effect of rain to the picture on the screen. This effect can be reduced, either by applying a matt or clear varnish, or by soaking the film in tepid water or in reagents causing a swelling of the gelatine.